This invention relates generally to processes for the decontamination of metallic nickel and, more specifically, to the decontamination of particulate nickel having a thin surface coating of metallic fluoride and containing actinide-metal fluorides. The invention is a result of a contract with the United States Department of Energy.
The invention was developed in response to problems encountered in attempts to decontaminate crushed metallic nickel scrap to produce purified saleable nickel ingots. The scrap was covered with a surface film of nickel fluoride and contained various radionuclides in the form of chemically active fluorides. The typical batch of the scrap contained the following: nickel fluoride (.about.1.2%); nickel oxide (.about.0.5%); uranium (.about.200 ppm); neptunium (.about.45 ppb); plutonium (.about.0.1 ppb); thorium (.about.0.15 ppb); and technetium (.about.60 ppm). The scrap was melted in a conventional electrical induction furnace having a rammed liner composed essentially of compacted and sintered ceramic. In a typical run, good decontamination was achieved in the melting operation; that is, with the exception of technetium, the radioactive contaminants were removed effeciently by transfer into slag and the furnace liner. However, serious operational problems occurred during melting. Some of the nickel fluoride reacted and/or decomposed, producing objectionable gaseous-fluoride emissions. Also, residual fluoride reacted with the furnace liner, reducing its life and creating a potential for serious safety hazards.
In an attempt to overcome these problems, a pre-treatment for the nickel scrap was developed to remove fluoride ions therefrom. The pre-treatment comprised contacting the scrap with steam to effect the following reaction (among others). EQU NiF.sub.2 +H.sub.2 O.fwdarw.NiO+2HF
This was accomplished in a rotary calciner, where the scrap was contacted with 20% steam in nitrogen at 1200.degree. F. for 30 minutes, at a flow rate of 8 stoichiometric quantities of steam. The pre-treatment destroyed the nickel-fluoride film effectively, as well as the other fluorides. The succeeding melting operation decreased the actinide-metal concentrations in the melt by satisfactorily large percentages. Unfortunately, the pre-treatment also increased the NiO content of the scrap to .about.4%. This resulted in the formation of excessive nickel oxide slag, which caused pouring problems, significant decreases in the service life of the liner, and a decrease in the value of the potentially saleable product nickel. Refinements of the steam-treatment system decreased the NiO concentration to nearly 2.1% (the theoretical minimum), but this did not decrease the NiO-associated problems to an acceptable degree. Pre-treatment of the scrap with methane resulted in a nickel metal having more uranium, neptunium, and plutonium contamination than the steam-treated scrap.